In recent years, a reduction in size and weight, an increase in power and an increase in efficiency have been required for permanent magnetic motors used in hybrid cars, hard disk drives or the like. Then, in realizing a reduction in size and weight, an increase in power and an increase in efficiency in the above-mentioned permanent magnetic motors, a reduction in film thickness and further improvement in magnetic characteristics have been required for permanent magnets buried in the permanent magnetic motors. Incidentally, as the permanent magnets, there are ferrite magnets, Sm—Co-based magnets, Nd—Fe—B-based magnets, Sm2Fe17Nx-based magnets and the like. In particular, Nd—Fe—B-based magnets having high coercive force are used as the permanent magnets for the permanent magnet motors.
Here, as a method for manufacturing the permanent magnet, a powder sintering method is generally used. In the powder sintering method as used herein, a raw material is first pulverized with a jet mill (dry pulverization) to produce a magnet powder. Thereafter, the magnet powder is placed in a mold, and press molded to a desired shape while applying a magnetic field from the outside. Then, the solid magnet powder molded to the desired shape is sintered at a predetermined temperature (for example, 1100° C. in the case of the Nd—Fe—B-based magnet), thereby manufacturing the permanent magnet.
Further, in the powder sintering method, when the raw material is pulverized with the jet mill, a slight amount of oxygen is usually introduced into the jet mill to control the oxygen concentration in nitrogen gas or Ar gas as a pulverizing medium to a desired range. This is because a surface of the magnet powder is forced to be oxidized, and the magnetic powder finely pulverized without this oxidation treatment ignites at the same time that it comes into contact with the air. However, most of oxygen in a sintered body obtained by sintering the magnetic powder subjected to the oxidization treatment is combined with a rare-earth element such as Nd to exist as an oxide in a grain boundary. Accordingly, in order to supplement the oxidized rare-earth element, it is necessary to increase the total amount of the rare-earth element in the sintered body. However, when the total amount of the rare-earth element in the sintered body is increased, there is a problem that the saturation magnetic flux density of the sintered magnet is decreased.
Accordingly, patent document 1 (JP-A-2004-250781) discloses a production method of, when a rare-earth magnet raw material is pulverized in a jet mill, recovering the pulverized magnet raw material in a rust preventive oil such as a mineral oil or a synthetic oil to form a slurry, wet molding this slurry in a magnetic field while performing deoiling, subjecting the molded body to deoiling treatment in vacuo, and performing sintering.